The invention relates to an electronic trip device for a multipole electrical circuit breaker protecting an electrical power system comprising:
a current sensor per pole supplying a secondary current proportional to a current flowing in an associated conductor of the power system protected by the circuit breaker,
a detection and rectifying circuit to which the secondary currents are applied and which supplies signals representative of the currents flowing in the conductors to be protected,
an electronic processing unit receiving the signals supplied by the detection and rectifying circuit and delivering a tripping order, with or without a time delay, when the currents flowing in the conductors to be protected or an earth fault current exceed preset thresholds.
Electronic circuit breakers comprising an earth fault protection function use earth fault current signal measurement or reconstitution circuits. FIG. 1 represents a circuit breaker of state-of-the-art type. An electrical power system 1 to be protected is composed of electrical conductors, generally corresponding to the three phases and neutral of an installation. Contacts 2 enable the current in the phase conductors to be established or interrupted. In the figure as in a great many installations, the neutral conductor is distributed without being broken. Current transformers T1, T2, T3 and TN associated with the different power system conductors transform the primary currents of high value into secondary currents compatible with electronic trip devices. The secondary currents are applied to the input of a phase, neutral and earth fault current rectifying and detection circuit 3. This circuit supplies signals representative of these currents to an electronic processing unit 4. A tripping order Dc1 produced by the processing unit 4 is applied to the input of a control relay 5 which actuates an opening mechanism 6 of the circuit breaker contacts 2.
In some state-of-the-art trip devices measuring of the earth fault current is achieved by means of a transformer Tp represented in FIG. 2. The transformer primary is formed by all the conductors of the power system 1 and a secondary winding supplies a current Ihs proportional to the earth fault current of the power system. A signal representative of this secondary current Ihs can be supplied by the voltage Vh1 at the terminals of a resistance Rh1 connected in parallel on the secondary winding of the transformer. FIG. 3 represents a diagram reconstituting a signal representative of an earth fault from the secondary currents of the current transformers TN, T1, T2 and T3. The sum of the secondary currents is obtained in a common conductor SI to which a first end of each of the current transformer secondary windings is connected, the second ends of these windings being connected directly to the rectifying circuit 3. The common conductor SI is connected via measuring means to the rectifying circuit. A current representative of the earth fault current, corresponding to the sum of the secondary currents of all the transformers, flows in the conductor SI. The measuring means can be formed by a resistance Rh2, the voltage Vh2 at the terminals of this resistance being representative of the earth fault current. Alternative measuring means are also represented in FIG. 3 in the form of a transformer TS1 whose primary winding is formed by the conductor SI. The measurement voltage Vh3 at the terminals of the secondary winding of the transformer TS1 is also representative of the earth fault current.
In trip devices using the rms value of the currents, total independence of the currents is necessary. A device according to the diagram of FIG. 4 enables this independence to be achieved. In this embodiment the two ends of the secondary winding of each current transformer (T1, T2, T3, TN) are connected to the rectifying circuit. The set of these secondary windings forms the primary of a transformer TSM which supplies at the terminals of its secondary winding a measurement voltage Vh4 representative of the earth fault current in the power system.
The signals Vh1, Vh2, Vh3, or Vh4 are then applied to the processing unit 4. In other trip devices, digital processing circuits calculate a value representative of the earth fault according to the amplitudes of the values of the phase and neutral currents as in U.S. Pat. No. 4,631,635.
State-of-the-art earth fault protection devices use, for reconstitution of the earth fault signal, electronic components occupying a large volume on the printed circuit boards. These components are often transformers, power diodes or resistors followed by shaping amplifiers. The cost and volume of these additional components are very high for electronic trip devices fitted on circuit breakers with low ratings.
Digital processing trip devices reduce the number of these power components but sample-and-hold circuits are required for simultaneous measurement of the current values. Moreover state-of-the-art algorithms for calculating the earth fault current value are not suitable for all phase and neutral current configurations, notably when these currents are not of pure sinusoidal shape.